In some known industrial processes, it is desirable or necessary to selectively remove a gas species from a flowing mixture of gas species. Such removal is typically carried out by passing the flowing mixture over a specifically adsorptive material deposited on a substrate having a high surface area. Typically, the adsorption capacity of the adsorptive material is finite and thus requires periodic reversal, or “regeneration”, to become effective again.
In some known adsorption cycles, the adsorber may be desorbed of gas simply by being flushed with air, whereby the adsorbed gases are removed and exhausted with the flushing air to atmosphere. In some of these cycles, the adsorber is required to be in a chemically reduced state and thus a second, reducing process must be applied to the adsorber after oxidation by air flushing.
A problem in the prior art is how to configure for continuous adsorbing operation a selective gas adsorber requiring both air flushing and adsorber reducing steps for complete regeneration of the adsorber.
An additional prior art problem is how to perform such an operation at elevated temperatures of the gases in the range of 500° C. to 1000° C. Providing continuous, non-degraded performance of a gas adsorber at such temperatures is a severe materials challenge.
A specific application of such an adsorption need is in the removal of H2S from the hydrocarbon reformate stream being supplied from a catalytic reformer to a solid oxide fuel cell (SOFC) stack. Several hydrocarbon fuels that are otherwise useful in such a system for generating hydrogen and carbon monoxide reformate fuels for the SOFC contain significant percentages of elemental sulfur and/or sulfur-based compounds which are converted in the reformer in H2S. Examples of such fuels are diesel fuel and JP8 jet fuel. Unfortunately, the anodes of present-day SOFCs are highly sensitive to H2S, and are substantially degraded by continuous exposure to H2S levels in the reformate of 2 parts per million (ppm) and thus in practice are limited to low-sulfur gasolines, alcohols, and de-sulfured natural gas as reformable fuels. (Because natural gas is colorless, odorless, and tasteless, a mercaptan is added before distribution to give it a distinct unpleasant odor that serves as a safety device by allowing it to be detected by human olfaction in the atmosphere in cases where leaks occur. In being reformed, the mercaptan also produces H2S.)
What is needed in the art is a continuous selective gas adsorption system having both oxidative and reductive capabilities of an adsorbing medium and being capable of sustained operation at high temperatures.
It is a principal object of the present invention to continuously remove hydrogen sulfide from a hydrocarbon reformate stream.